This invention relates to a screw pump which transports a fluid from an intake port to a discharge port by the turning action of a screw assembly in a pump casing, the assembly forming fixed-capacity sealed chambers.
In this type of pump, normal manufacturing tolerances result in a small amount of leakage inside the pump casing, and this leakage causes the pressures of the multiple sealed chambers arranged along the screw assembly to gradually increase towards the discharge port. Thus, a differential pressure between chambers or closures, approximately equivalent to the value obtained by dividing the discharge pressure by the number of sealed chambers, is generated between the discharge port and the final sealed chamber which is adjacent to the discharge port.
Furthermore, a liquid being pumped inside the sealed chambers contains minute air bubbles, and/or when the sealed chamber closest to the intake port end is formed, the resulting empty space becomes filled with free gas and air bubbles, if the sealed chamber closes when it is not sufficiently filled with liquid. Because these air bubbles are suddenly exposed to high pressure when the final sealed chamber at the discharge end is opened, they are compressed and collapsed, thus causing cavitation erosion of the metal surfaces. This also generates noise and vibration, and further, it allows part of the pressurized fluid to flow from the discharge port back to the final sealed chamber. This phenomenon occurs each time the final sealed chamber opens to the discharge side, and thus results in a pulsation of the discharge port's pressure and discharge flow. The degree of this pulsation increases as the differential pressure increases between the discharge port and the final sealed chamber just prior to its opening.
For these reasons, screw pumps such as those described in Japanese Patent Publications No. 36-9922 published July 7, 1961 and No. 39-17791 published Aug. 25, 1964 have been proposed in order to suppress the aforementioned pulsation, noise, and vibration. The screw pump described in Patent Publication No. 36-9922 is constructed with a screw assembly consisting of a drive screw or power rotor having protruding or male threaded sides and two driven screws or idler rotors having concave threaded sides which are closely meshed with the drive screw. The screw assembly is closely fitted into a sleeve, thus forming sealed chambers. In this screw pump, as shown in FIG. 3 of the publication, an indentation 13 is provided on one side of the threads of at least one of the screws, for example the main or drive screw 1a. This indentation thus forms a spiralling passage through the entire screw assembly causing the pressure of the fluid chambers to gradually increase from the intake port pressure to the discharge port pressure. This in turn causes the air bubbles to be compressed gradually, thus making it possible to avoid the sudden collapsing of the air bubbles at the discharge port and to obtain smoother operation.
In the screw pump described in Patent Publication No. 39-17791, as shown in FIGS. 2 and 4, using some point between the intake end and the discharge end as a reference point, one or more V-shaped intersections of the passage housing the main screw and the passages housing the driven screws, inside the sleeve, are either cut at a slanting angle or are cut progressively larger either in the direction of the discharge end or in both directions thus forming channels which guide the fluid from the discharge end toward the intake end. This arrangement causes the pressure inside each sealed chamber to progressively increase as the chamber moves closer to the discharge end, thus gradually eliminating most of the air bubbles trapped inside the sealed chambers.
However, these solutions have not been entirely satisfactory. Because the screw pump described in Patent Publication No. 36-9922 requires a spiralling indentation to be formed along the entire length of the screw, and the screw pump described in Patent Publication No. 39-17791 requires tapered channels to be formed along the entire length, or almost the entire length, of the sleeve, fabrication of these pumps is a complicated process and production costs are high. Furthermore, fluid constantly flows from the discharge port toward the intake end, and there is an increased amount of leakage inside the pump. Thus, because there is a considerable amount of energy loss and because the amount of leakage increases as the pressure increases, these prior art pumps are not well suited for high-pressure operation.
It is the purpose of this invention to avoid the drawbacks described above. The purpose of this invention is to, by a relatively simple means, gradually increase the pressure of only the final sealed chamber adjacent to the open chamber in order to reduce the differential pressure between the final sealed chamber and the discharge port, thus greatly reducing the amount of pulsation, noise, and vibration, and thereby providing a screw pump which consumes less energy and which is well suited to high-pressure operation.